In the manufacture of semiconductor devices and other products, ion implantation systems are used to implant dopant elements into work pieces (e.g., semiconductor wafers, display panels, glass substrates). These ion implantation systems are typically referred to as “ion implanters”.
FIG. 1 illustrates one example of an ion implantation system 10 having a terminal 12, a beamline assembly 14, and an end station 16. Generally speaking, an ion source 18 in the terminal 12 is coupled to a power supply 20 to ionize a dopant gas and form an ion beam 22. The ion beam 22 is directed through a beam-steering apparatus 24, and out an aperture 26 towards the end station 16. In the end station 16, the ion beam 22 bombards a work piece 28 (e.g., a semiconductor wafer, or a display panel), which is detachably mounted to an electrostatic chuck 30. Once embedded into the lattice of the workpiece 28, the implanted ions change the physical and/or chemical properties of the workpiece. Because of this, ion implantation is used in semiconductor device fabrication and in metal finishing, as well as various applications in materials science research.
Absent countermeasures, during the ion implantation process energy can build up on the workpiece 28 in the form of heat as the charged ions collide with the workpiece. This heat can warp or crack the workpiece, which may render the workpiece worthless (or significantly less valuable) in some implementations.
In addition, even if the workpiece is not rendered worthless, this undesired heating can cause the dose of ions delivered to differ from the dosage desired, which can alter the functionality from what is desired. For example, if a dose of 1×1017 atoms/cm3 are desired to be implanted in an extremely thin region just below the outer surface of the workpiece, unexpected heating could cause the delivered ions to diffuse out from this extremely thin region such that the dosage actually achieved is less than 1×1017 atoms/cm3. In effect, the undesired heating can “smear” the implanted charge over a larger region than desired, thereby reducing the effective dosage to less than what is desired. Other undesirable effects could also occur.
In other instances it might be desired to implant at a temperature below ambient temperature, to allow the chuck to be cooled to very low temperatures. For these and other reasons, cooling systems have been developed. Although cooling systems are known in some respects, such as in plasma processing apparatuses, it is extremely difficult to integrate a vapor cooling system into an ion implanter due to the mechanical density of components near the workpiece. For example, electrostatic chucks in ion implanters are often considerably more complicated that those used in run-of-the-mill plasma processing apparatuses. Consequently, the inventors have developed techniques for cooling electrostatic chucks in ion implantation systems, which can reduce undesired heating of workpieces undergoing implantation.